Beginner here with quite a basic question. How many hits can a single armor unit sustain (with a single tank on the hex)?

One armor die roll and they're dead?

I just thought that really nerfs their power when opposing infantry have terrain defense - armor would only get 1 die to attack with and could be killed with the single hit, whereas infantry can sustain 4 hits.

Yes, it's one hit per figure, so in the case you describe, with 1 tank figure remaining on a hex, any die showing either a tank symbol or a grenade symbol (in the simplest of settings) would destroy the unit and gain the victor a medal.

This presents a 1 in 3 chance of being hit.

(Again, this is in the most standard context - backed up with nowhere to retreat for exapmle would increase these odds, as flags suddenly count as hits).

In standard terrain, infantry are more vulnerable to hits. (1in2)

As far as how all this affects a tanks' strength ..

This is simply how the game is played, with tanks generally harder to hit than say infantry, BUT infantry benefitting significantly from terrain defense.

Bear in mind, tanks USUALLY start with 3 figures, or sometimes more, (sometimes even less to represent a weaker strength tank scenario).

Learning how and when to use your tanks to great effect is all part of the fun of Memoir44!

Rolling up to a town, parking outside and firing blindly into it is not maybe the tactic you would want to employ with your tanks, is all I'm saying!

Yep as a general rule, Infantry are better at attacking built up areas where tanks are more vunerable. But Tanks are excellent attacking out in the open (full dice plus the chance of an armoured overrun) where as infantry are at the mercy of tanks in open.